[BBC-Micro] TUBE chip, accessing 'Parasite' side

Jonathan Graham Harston jgh at arcade.demon.co.uk
Sat Mar 22 19:52:00 GMT 2008

Johan Heuseveldt wrote:
> Some of the technical info surprises me. The sector size is 256
> bytes, but only 252 bytes for data. The other 4 bytes are for
> system use! Random access to files isn't so flexible; one byte
That's correct. Sectors are 256 bytes. Sectors are accessed as a
linked list. 252 bytes are the file data, and four bytes are the
sector number to fetch for the next 252 bytes. It makes random
access a bit fiddly, as a sector chain has to be followed all the
way through to the point you need. PTR#chn=value doesn't just
translate into sector=start+value DIV 256.
> The current design is not FLEX compatible anyway, as there is no
> appropiate MMU present; intentionaly though. Not sure I like
You don't need an MMU to be FLEX compatible, it can be a single
64K single-bank FLEX system.
J.G.Harston - jgh at arcade.demon.co.uk - mdfs.net/User/JGH
HADFS System Resources - http://mdfs.net/Software/HADFS

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